Power MOSFETs have source and drain regions formed within the channel region of a semiconductor substrate, and a gate electrode formed therebetween. In the case of an N-type MOSFET, the channel region is coupled to the source region with lowest potential so that a floating state does not occur. Consequently, a parasitic body diode is formed from the source region and the channel region coupled thereto, to the drain region. That is, in a normal circuit, the current path in the direction from the drain region toward the source region in an N-type MOSFET is turned on and off, but a body diode is continuously formed in the current path from the source region toward the drain region, and current is never shut off.
Hence in order to form a bidirectional switch using a power MOSFET, it has been necessary to combine two power MOSFETs, resulting in increased costs.
On the other hand, in order to prevent overcharge and overdischarge of lithium ion batteries, in addition to battery cells in a battery pack, a charge/discharge protection device to prevent overcharge and overdischarge is provided. This charge/discharge protection device has a bidirectional switch. The bidirectional switch must perform on/off switching of current in both directions, and is configured using two power MOSFETs.
It has been proposed that SITs (Static Induction Transistors) be used in bidirectional switch elements, to perform current on/off switching in both directions. One example of this is Japanese Patent Application Laid-open No. 2001-251772. However, such a bidirectional switch element takes only two states, to switch current on or off in both directions.
In USB equipment, in addition to its own internal power supply, power is also supplied to the equipment via a USB bus. Hence it is necessary to switch between the internal power supply and the USB bus power supply, and a bidirectional switch element is necessary.